The present invention relates to bandpass and band reject filter systems using a feedback loop particularly ones synthesizing a cancelling signal.
Often, analog signals are transmitted having a beacon frequency, i.e. a frequency tone, contained within them which is used as a frequency reference for one or another purposes. Once this purpose is satisfied, it is often necessary or desirable to remove this beacon frequency from the analog signal before further processing of this signal.
Removal of this tone requires a narrow band rejection filter, i.e. a notch filter. For a practical notch filter, it is impossible to remove a tone frequency without distorting the remaining frequencies in the analog signal. The narrower the frequency notch, i.e. frequency rejection band, of a notch filter, the smaller the distortion will be of the remaining frequencies in the analog signal. Passive notch filters have a minimum rejection bandwidth for a given attenuation requirement at the notch frequency.
Among passive notch filters, crystal notch filters will provide the smallest rejection bandwidth for a given amount of attenuation at the frequency. However, the rejection bandwidth of even an ideal crystal notch filter can be too wide to meet the bistortion requirements needed to preserve the remaining frequencies in the analog signal from which the tone is to be removed. An increase in the rejection bandwidth beyond that required by an ideal crystal notch filter comes about due to some inevitable mistuning of the crystal notch filter and to temperature and component aging changes causing a drift in the filter notch frequency. Finally, the frequency of the tone to be cancelled also often drifts and the rejection bandwidth of a crystal notch filter must be wide enough to encompass the frequency of the tone to be cancelled despite its drift since the notch frequency of a crystal notch filter is not tunable.
As is well known, band rejection filters are used in the electronics arts beyond the instance discussed in the foregoing to remove wider bands of frequencies in analog signals for various reasons. This is also true of bandpass filters which are used to pass therethrough only a selected narrow or wide band of frequencies in an analog signal as may be required in a particular application. Precise filters capable of performing these filtering functions around a central frequency of a substantial magnitude are quite desirable.